Method and apparatus for improved ellipsometric measurement of ultrathin films

ABSTRACT

A method for implementing ellipsometry for an ultrathin film includes directing a polarized light beam incident upon a sample surface, receiving an initial reflected beam from the sample surface and redirecting the initial reflected beam back upon said sample surface one or more times so as to produce a final reflected beam. The final reflected beam is received through an analyzer and at a detector so as to determine characteristics of the ultrathin film.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. Ser. No.11/688,366, filed Mar. 20, 2007, which is a continuation application ofU.S. Ser. No. 10/904,462, filed Nov. 11, 2004, the disclosures of whichare incorporated by reference herein in their entirety.

BACKGROUND

The present invention relates generally to semiconductor devicemanufacturing, and, more particularly, to a method and apparatus forimproved ellipsometric measurement of ultrathin films.

Ellipsometry is an optical technique that uses polarized light to probethe properties of a sample. One of the most common applications ofellipsometry is the analysis of thin films. Through the analysis of thestate of polarization of the light that interacts with a sample,ellipsometry can yield certain information about the properties of suchfilms. For example, depending on what is already known about the sample,the technique can probe a range of properties including the layerthickness, index of refraction, morphology, or chemical composition.

Generally, optical ellipsometry may be defined as the measurement of thestate of polarized light waves. An ellipsometer measures the changes inthe polarization state of light when it interacts with a sample. Themost common ellipsometer configuration is a reflection ellipsometer,although transmission ellipsometers are also sometimes used. If linearlypolarized light of a known orientation is reflected or transmitted atoblique incidence from a sample surface, then the resultant lightbecomes elliptically polarized. The shape and orientation of the ellipsedepends on the angle of incidence, the direction of the polarization ofthe incident light, the wavelength of the incident light, and theFresnel properties of the surface.

The polarization of the light is measured for use in determining certaincharacteristics of the sample. For example, in one conventional nullellipsometer, the polarization of the reflected light may be measuredwith a quarter-wave plate, followed by an analyzer. The orientation ofthe quarter-wave plate and the analyzer are varied until no light passesthough the analyzer (i.e., a null is attained). Based on theseorientations and the direction of polarization of the incident light, adescription of the state of polarization of the light reflected from thesurface may be calculated and the sample properties deduced.

Two characteristics of ellipsometry make its use particularly attractivein the field of semiconductor manufacturing. First, since ellipsometryis a nondestructive technique, it is suitable for in situ observation ofa sample. Second, the technique is extremely sensitive in that smallchanges of a film may, in certain instances, be measured down to asub-monolayer of atoms or molecules. Accordingly, ellipsometry has beenwidely used in areas such as physics, chemistry, materials science,biology, metallurgical engineering and biomedical engineering, to name afew. At the same time, however, advances in microelectronics fabricationare rapidly surpassing current capabilities in metrology. In order toenable the continued scaling of future generations of microelectronics,advances in specific metrology capabilities must also follow suit, suchas the ability to measure the properties of ultra-thin films (e.g.,thicknesses on the order of 20 angstroms or less) over sub-micronlateral dimensions.

Unfortunately, existing ellipsometry systems have difficulty inmeasuring and distinguishing between certain characteristics (e.g.,index of refraction, thickness, etc.) of ultrathin films having varyingoptical properties. In the past, certain optical properties (such asmaterial composition) have been assumed for thin films such as gatedielectrics where the dielectric material utilized was an oxide ornitride material, for example. However, with the use of more advancedultrathin gate dielectrics, the traditional assumptions as to thecomposition of the dielectric material are no longer reliable for use inellipsometric measurements. In particular, these ultrathin films do notproduce enough of a phase shift on an incident beam to adequatelydistinguish between film thickness and film composition. Thus, a needexists for improving conventional ellipsometric techniques so as to beable to reliably obtain the desired measurements of advanced ultrathinfilms.

SUMMARY

The foregoing discussed drawbacks and deficiencies of the prior art areovercome or alleviated by a method for implementing ellipsometry for anultrathin film. In an exemplary embodiment, the method includesdirecting a polarized light beam incident upon a sample surface,receiving an initial reflected beam from the sample surface andredirecting the initial reflected beam back upon said sample surface oneor more times so as to produce a final reflected beam. The finalreflected beam is received through an analyzer and at a detector so asto determine characteristics of the ultrathin film.

In another embodiment, a method for determining film thickness andcomposition for an ultrathin film formed on a semiconductor substrateincludes directing a polarized light beam incident upon a surface of theultrathin film and receiving an initial reflected beam from saidultrathin film surface. The initial reflected beam has a phase shiftwith respect to the beam incident upon the ultrathin film surface. Theinitial reflected beam is redirected back upon the sample surface for aplurality of iterations, wherein the phase shift is increased with eachof the iterations, so as to produce a final reflected beam. The finalreflected beam is received through an analyzer and at a detector so asto determine the film thickness and composition of the ultrathin film.

In still another embodiment, an ellipsometry apparatus for determiningcharacteristics of an ultrathin film includes a light source and apolarizer configured to direct a polarized light beam incident upon asample surface. The apparatus further includes a means for receiving aninitial reflected beam from the sample surface and redirecting theinitial reflected beam back upon the sample surface one or more times soas to produce a final reflected beam. An analyzer is configured toreceive the final reflected beam therethrough, and a detector isconfigured to determine characteristics of the ultrathin film.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the exemplary drawings wherein like elements are numberedalike in the several Figures:

FIG. 1 is a schematic diagram of an existing reflection ellipsometerapparatus;

FIG. 2 is a schematic diagram of an ellipsometer apparatus specificallyconfigured to repeatedly direct the initially reflected light back uponan ultrathin film surface, in accordance with an embodiment of theinvention; and

FIG. 3 is a schematic diagram of an alternative embodiment of theellipsometer apparatus of FIG. 2.

DETAILED DESCRIPTION

Disclosed herein is an improved method and apparatus for ellipsometrythat will aid in the measurement and characterization of ultrathinfilms, such as those used in advanced semiconductor manufacturing.Briefly stated, the invention embodiments overcome the disadvantage ofhaving minimal phase shift information by repeatedly directing theinitially reflected beam back onto the sample so as to cumulativelyincrease the phase shift effect created by the ultrathin film. In sodoing, the accumulated phase shift information of the repeatedlyreflected beam provides increased reliability of the accuracy of themeasured parameters of interest. It will be appreciated that theembodiments described hereinafter are not only applicable to the fieldof semiconductor manufacturing, but are also applicable to other areaswhere ultrathin films are measured and analyzed.

Referring initially to FIG. 1, there is shown a schematic diagram of anexisting reflection ellipsometer apparatus 100 for performingellipsometric measurements on a sample 102, in which an ultrathin layer104 is formed upon a substrate 106. As is shown, the apparatus 100includes a polarizer portion 108 and an analyzer portion 110. Thepolarizer portion 108 includes a light source 112 such as a laser (e.g.,a 632.8 nm helium/neon laser or a 650-850 nm semiconductor diode laser)and a polarizer 114, which provides a state of polarization for anincident light beam 116 at a non-normal angle with respect to thesurface of the sample 110 (e.g., about 20° with respect to the samplesurface plane, or about 70° with respect to the normal of the surfaceplane). The incident light beam 116 is typically linearly polarized (asis depicted in FIG. 1) with finite field components E_(p) and E_(s) inthe directions parallel and perpendicular to the surface plane.(However, in other systems, the incident beam 116 may also beelliptically polarized light or circularly polarized light.)

Upon reflection of the incident light beam 116 off the ultrathin layer104 of the sample 102, the initial linear polarization of the reflectedlight beam 118 is changed to a slight elliptical polarization due to theproperties of the ultrathin layer 114, in accordance with the Fresnelequations. The reflected light beam 118 is then analyzed with theanalyzer portion 110 of the ellipsometer apparatus 100. In particular,the analyzer portion 110 includes an analyzer 120 (e.g., a secondpolarizer generally crossed with the first polarizer 114) and a detector122.

In order to measure the polarization of the reflected light beam, theoperator may change the angle of the polarizer 114, the analyzer 120,and/or other additional optical components until a minimal signal isdetected. For example, the minimum signal is detected if the light 118reflected by the sample 102 is linearly polarized, while the analyzer120 is set so that only light with a polarization that is perpendicularto the incoming polarization is allowed to pass. The angle of theanalyzer 120 is therefore related to the direction of polarization ofthe reflected light 118 if the minimum condition is satisfied. Theapparatus is “tuned” to this null condition (e.g., generallyautomatically under computer control), and the positions of thepolarizer 114, the analyzer 120, and the incident angle of the light 116relative to the plane of the sample surface are used to calculate thefundamental quantities of ellipsometry; i.e., the so called (psi (Ψ),delta (Δ)) pair, given by the expression:

$\frac{r_{p}}{r_{s}}\tan \; {\Psi \left( ^{j\Delta} \right)}$

where r_(p) and r_(s) are the complex Fresnel reflection coefficientsfor the transverse magnetic and transverse electrical waves of thepolarized light, respectively. Thus, from the ellipsometry pair (Ψ, Δ),the thickness and index of refraction of a thin film may be determined.It will also be recognized that various other ways of analyzing thereflected light are also possible. For example, one possible alternativewould be to vary the angle of the analyzer 120 to collect polarizationinformation.

As mentioned previously, however, ultra-thin films impart very littlephase shift with respect to the incident light beam. Because themeasured film characteristics (thickness and optical properties) arebased on the extent of the phase shift imparted by the ultrathin film104, the ellipsometer apparatus 100 of FIG. 1 is not suited for reliablecharacterization thereof due to this minimal phase shift. Therefore, inaccordance with an embodiment of the invention, FIG. 2 illustrates anellipsometer apparatus 200 specifically configured to repeatedly directthe initially reflected light back upon the ultrathin film surface usingreflective devices that maintain the phase shift of the initiallyreflected light. In this manner, the multiple passes will cumulativelyincrease the phase shift such that it can be more confidently measured.Moreover, the angle of incidence and orientation variations may beintroduced, which will provide additional information not normallyavailable with a classic ellipsometer or spectroscopic ellipsometer.

More specifically, the ellipsometer apparatus 200 includes a pluralityof reflective devices 202 a-202 f (e.g., mirrors having a minimum or afixed/known phase shift) configured within the optical path prior to theanalyzer 120. A first mirror 202 a receives the initial reflected beam118 a, redirecting it to a second mirror 202 b in a non-interactingmanner (indicated by a dashed line) with respect to the film 104 at apoint near the origin of the incident light beam 116. The second mirror202 b then redirects the light back off of the film 104, atapproximately the same point 204 as the incident light beam 116. Thisredirected reflection process is then repeated using subsequent pairs ofmirrors (e.g., 202 c/202 d, 202 e/202 f) until a suitable amount ofiterations of phase shifting of the beam is achieved.

It should be appreciated that the schematic depiction of FIG. 2 is notto scale, and that the spacing, arrangement, and particular number ofthe mirrors 202 a-202 e are shown for illustrative purposes only. Itwill further be understood that in the plan view of FIG. 2, the mirrors202 a-202 e are situated above the plane of the film 104 and that thereflected beams are incident upon the film 104 at about the location ofthe point 204 of reflection. In the embodiment illustrated, theindividual mirrors are oriented such that the redirected incident beamsare incident at the original point 204 of reflection. The surfaces ofmirrors 202 a-202 e may be further be curved in a manner such that thereflected beams are refocused to the initial point 204 of reflection.

It will also be appreciated that, in addition to using an individualmirror for each of the reflections, a single mirror on opposite sides ofthe sample could also be used, provided the mirror geometry isappropriately configured. (In other words, individual mirrors 202 a, 202c and 202 e could be functionally implemented as a first mirror on oneside of the wafer 104, while mirrors 202 b, 202 d and 202 f could befunctionally implemented as a second mirror on the opposite side of thewafer 104.) In any case, a final reflected beam 118 b (after alliterations) is directed to the analyzer 120 and detector 122 formeasurement of the ellipsometric parameters. Again, the mirrors 202a-202 e are configured such that the polarization of the final reflectedbeam 118 b is substantially the same is the polarization of thereflected beam 118 a, only with the effect of the phase shift beingaccumulated such that it is more easily measured.

The specific number of mirrors or, more generally, the number ofreflections may be adjusted (i.e., automatically inserted or removedwhere needed) such that the amount of the phase shift is controllablefor maximum confidence in the measurement process. In addition, themirrors may also be removed entirely in order to return to a classicalellipsometer configuration. Still another advantage of the embodiment ofFIG. 2 stems from the fact that the angle of incidence that each mirrorintroduces in reflecting the light back off the sample may be variedsuch that additional information (e.g., ellipsometric parameters as afunction of angle of incidence) can be obtained.

Depending upon the specific application, measurement and filmcomposition involved, an appropriate number of reflective passes may beimplemented. For example, with currently manufactured gate oxides, atotal of 10 passes (reflections) may be suitable for accurate filmthickness and composition determination. On the other hand, for a simplepass/fail test, a fewer number of reflections may be appropriate.

Finally, FIG. 3 illustrates an alternative embodiment of an ellipsometerapparatus 300, in which the path of the reflected beam is adjustablyrerouted so that it does not follow the same direction as the originalpath. This approach may be useful, for example, for measuring materialoptical properties such as birefringence. As is shown in the exemplaryembodiment of FIG. 3, the path of the initial reflected beam 118 a ischanged from an x-direction to a y-direction (orthogonal) for the finalreflected beam 118 b through the placement of various mirrors 202. Itwill be appreciated, however, that other directions across the samplemay be realized, for several iterations where desired.

While the invention has been described with reference to a preferredembodiment or embodiments, it will be understood by those skilled in theart that various changes may be made and equivalents may be substitutedfor elements thereof without departing from the scope of the invention.In addition, many modifications may be made to adapt a particularsituation or material to the teachings of the invention withoutdeparting from the essential scope thereof. Therefore, it is intendedthat the invention not be limited to the particular embodiment disclosedas the best mode contemplated for carrying out this invention, but thatthe invention will include all embodiments falling within the scope ofthe appended claims.

1. A method for determining film thickness and composition for anultrathin film formed on a semiconductor substrate, the methodcomprising: directing a polarized light beam incident upon a surface ofthe ultrathin film; receiving an initial reflected beam from saidultrathin film surface, said initial reflected beam having a phase shiftwith respect to said beam incident upon the ultrathin film surface;redirecting said initial reflected beam back upon said sample surfacefor a plurality of iterations, wherein said phase shift is increasedwith each of said iterations, so as to produce a final reflected beam;and receiving said final reflected beam through an analyzer and at adetector so as to determine the film thickness and composition of theultrathin film.
 2. The method of claim 1, wherein said redirecting saidinitial reflected beam back upon said ultrathin film surface isimplemented through one or more reflective surfaces.
 3. The method ofclaim 2, wherein said one or more reflective surfaces compriseindividual mirrors.
 4. The method of claim 2, wherein said one or morereflective surfaces comprise a first mirror on one side of said samplesurface and a second mirror on an opposite side of said sample surface.5. The method of claim 3, wherein said mirrors comprise curved surfacesconfigured for said redirecting said initial reflected beam back uponsaid ultrathin film surface.
 6. The method of claim 1, furthercomprising redirecting said initial reflected beam back upon saidultrathin film surface along a substantially constant direction.
 7. Themethod of claim 1, wherein said redirecting said initial reflected beamback upon said ultrathin film surface is implemented at least once in anorthogonal direction with respect to the direction of said initialreflected beam so as to measure birefringence of the ultrathin film. 8.The method of claim 3, wherein said individual mirrors are adjustablewith respect to the positioning thereof.